1. Field of the Invention
This invention relates to a gas sensor, and more particularly to a shock-resistive gas sensor for detecting a gaseous ingredient or its concentration.
2. Related Art Statement
It has been known to detect the presence and concentration of a gas in air by a gas sensor which uses a gas-sensitive element made of an oxide semiconductor, such as tin oxide (SnO.sub.2), zinc oxide (ZnO), titania (TiO.sub.2), cobalt oxide (CoO), and the like, whose electric resistance varies upon exposure to the gas. To simplify the structure of such gas-sensitive element for improving the productivity thereof, the so-called hybrid techniques has been developed; for instance, by printing both the gas-sensitive element and electrodes therefor in the form of thick films on an electrically insulating ceramic substrate.
It is generally difficult to mount directly the ceramic substrate carrying the gas-sensitive element onto a device in a combustion environment or in an exhaust gas duct for sensing a specific gas therein.
To hold such a gas sensor in position for actual measurement, it has been practiced heretofore to put it in a housing which can be readily fixed at a location for detecting the object gas. However, certain difficulties have been experienced: for instance, that the gas sensor with the gas-sensitive element formed on the ceramic substrate has smooth surface with little undulations and it has been difficult to determine which part of the sensor be fixed to the housing with or without a spacer therebetween; and that, when a spacer is used between the housing and the gas sensor, the spacer tends to contact the gas-sensitive element and no harm to it during the assemblying process, because the gas-sensitive element projects from the substrate for sensing the gas while the spacer keeps rather tight contact with the substrate for ensuring the secure holding.
To overcome such difficulties, it has been proposed to form a staircase-like step on the substrate carrying the gas-sensitive element, so that the substrate is held in position by engaging the staircase-like step against a holder or spacer in the housing. For instance, the gas-sensitive element is formed at a tip portion of an elongated substrate while the staircase-like step is formed at root portion of such elongated substrate, whereby the root portion of the substrate is firmly held in position within the housing so as to extend the gas-sensitive element into the atmosphere to be measured. The root portion may be fastened to the housing by using suitable fillers or the like. The housing holding the substrate with such staircase-like step can be easy mounted on the device where the gas is to be sensed. However, the staircase-like step of the substrate has a shortcoming in that it is rather weak against mechanical shock, such as impact during transportation or shock due to inadvertent dropping, and rather frequent breakage has been experienced in the substrate of such gas sensor due to mechanical shocks during handling.